Button cells usually have a housing comprising two housing halves, a cell cup and a cell lid. These can be produced, for example, from nickel-plated deep-drawn metal sheets as punch-drawn parts. The cell cup usually forms the positive pole, the cell lid forms the negative pole. Such a housing can contain a wide variety of electro-chemical systems, for example, zinc/manganese dioxide, primary and secondary lithium systems or secondary systems such as nickel/cadmium or nickel/metal hydride.
The liquid-tight closure of such button cells is typically effected by flanging the edge of the cell cup over the edge of the cell lid in connection with a plastic ring arranged between the cell cup and the cell lid which simultaneously serves as a sealing element as well as for electrical insulation of the cell cup and of the cell lid. Such button cells are described, for example, in DE 31 13 309.
As an alternative, it is also possible to produce button cells where, in the axial direction, the cell cup and the cell lid are exclusively held together only by a force-fit connection and which correspondingly do not have a flanged cup edge. Such button cells as well as a method for producing the same have been described, for example, in DE 10 2009 017 514.
Regardless of the various advantages that can be provided by button cells not having such flanging, they are yet less stress resistant in the axial direction than comparable button cells having a flanged cup edge, in particular in terms of axial mechanical loads which can be due to reasons in the interior of the button cell. For example, the electrodes of rechargeable lithium-ion systems are always subject to volume changes during charging and discharging processes. The axial forces involved naturally tend to cause leaks comparatively more likely in the case of button cells without a flanging than in the case of button cells with a flanging.
Windings consisting of flat electrode and separator layers can be produced quite easy according to known methods (e.g., see DE 36 38 793), for example, by applying, in particular laminating, the electrodes in a planar manner, particularly in the form of strips, onto a separator present as an endless sheet. Generally, the assembly composed of electrode and separator is wound up onto a so-called “winding mandrel.” After stripping the winding off the mandrel, an axial cavity remains in the center of the winding with the result that the winding can possibly relax into the cavity. The result thereof can be problems in electrical contacting of the electrodes and the metal housing halves.
Severe safety problems can occur in the case of lithium-ion or lithium-polymer cells, for example, caused by current pulses as can be caused by an external short-circuit, or by overcharging. Lithium-ion cells often include a graphite-containing anode and a lithium-cobalt-oxide-based cathode. During charging, lithium ions are released from the lithium-cobalt-oxide and intercalated into the graphite layers of the anode. If such a cell is overcharged, in particular to a voltage of more than 4.2 V, it is possible that more lithium ions are released than can be taken up by the graphite layers of the anode. As a result, highly reactive metallic lithium precipitates on the surface of the anode. If the charging process is further continued and the voltage is correspondingly further increased, in particular to a level significantly higher than 4.2 V, parts of the electrolyte in the cell may decompose and result in gas evolution. Furthermore, the lithium-cobalt-oxide structure becomes more and more instable due to the progressing release of the lithium until it finally breaks down by releasing oxidants (oxygen). Under certain conditions, these processes may lead to a significant heating of the cell which can result in an explosive combustion.